Does being a nobody mean God can't use me?

No. Even unseen or unknown members of the body of Christ are important, like the little toe. If you are in Christ, you are a somebody, not a nobody. God gives gifts differing according to His grace, whether great or small, and He is the one building the body.

Les teaches that the body of Christ has members as unseen and unknown as the little toe on your foot, yet they are absolutely important. Others are more visible, like Billy Graham and famous evangelists, but they are no more important. So never sit back and say God can't use me because I'm a nobody. If you're in Christ, you're a somebody. Les illustrates this with a listener from Minneapolis who corrected him when he called himself a nobody, saying, 'No you're not, you're in Christ, and if you're in Christ, you're somebody.' Then in Romans 12, Paul writes that we are many members yet one body, with gifts differing according to the grace given. God is in control of gift giving; He imparts your particular gift, whether great or small, and He is building the body. Since the New Testament had not yet come together, God promoted His Word through gifted men who spoke forth the Word of God, which is what prophecy means—not telling the future like Isaiah did.

Romans 12:5

So we, being many, are one body in Christ, and every one members one of another.

What do you say to someone who thinks God can't use them because they're a nobody?

The response is that in Christ they are not a nobody but a somebody. Every member of the body of Christ matters, even the unseen and unknown, like the little toe. God gives gifts differing according to His grace, and He is the one who builds the body.